The present invention relates in general to the field of radio communication and, in particular, to methods and means for providing a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other and have no broadcast control channel.
There are a number of equipments that have some sort of radio communication means. By xe2x80x9cradio unitxe2x80x9d is meant all portable and non-portable equipment intended for radio communication with a radio communication system. Examples of such radio units are mobile phones, cordless phones, pagers, telex, electronic notebooks, PCs and laptops with integrated radios, communicators, computers, wireless head sets, wireless printers, wireless keyboards or any other electronic equipment using a radio link as a mean of communication. These equipments can be used with any type of radio communication system, such as cellular networks, satellite or small local radio networks. They can also communicate directly with each other without using any system.
Cellular radio communication systems are commonly employed to provide voice and data communications to a plurality of radio units or subscribers.
Examples of such cellular radio communication systems are e.g. AMPS, D-AMPS, GSM, and IS-95 (CDMA). These systems generally include a number of base stations serving portable radio units, one or more base station controllers (BSC) and at least one mobile switching centre (MSC) or similar. All radio transmissions in the system are made via a specific radio interface that enables radio communication between the portable radio units and the base stations.
The cellular radio communication system covers a certain geographical area. This area is typically divided into cells or regions. A cell typically includes a base station and the radio units with which the base station is in communication. The cell associated with the particular base station with which a radio unit is communicating is commonly called the serving cell.
To each cell one or more voice/data and/or traffic/control channels are allocated. Note that xe2x80x9cchannelxe2x80x9d may refer to a specific carrier frequency in an analogue system, e.g. AMPS, a specific carrier/slot combination in a hybrid TDMA/FDMA system, e.g. GSM or one or more assigned codes in a DS-CDMA system.
The cellular radio communication system usually provides a broadcast channel on which all radio units can listen to system information from base stations or measure signal strength and/or signal quality at regular intervals. Such a channel is called Broadcast Control Channel in GSM and Page or Access Channel in D-AMPS.
The process of changing cells during a call is often called a handover or handoff. As soon as one of the neighbouring cells is considered to have a better signal strength/quality than the serving cell, e.g. by signal measurements on the broadcast channel, a handover is made to that particular neighbouring cell.
The ability to move around, changing cells and connections over the radio interface when the radio unit is switched on or is in some kind of stand by mode but not engaged in a call is called roaming. When the radio unit is roaming it listens to the broadcast channel for information about the system e.g. in which specific area of the system the radio is presently located.
Today, a number of low-power, low-cost radio interfaces between radio units and their accessories are being developed. The intention is to replace the cables or infrared links, e.g. between a computer and a printer, with a short-range radio link (a wireless link) forming a local radio network.
A suitable frequency band for such a radio-interface is the 2,4 GHz ISM band (the Industrial-Scientific-Medical band) which ranges from 2,400-2,483 GHz in the US and Europe and from 2,471-2497 GHz in Japan. This frequency band is globally available, licence-free and open to any radio system.
There are some rules each radio system has to follow if they are to use this ISM band, e.g. in the ETSI standard ETS 300328. Synchronisation between different transmitters in a radio system using the ISM band is not allowed. Synchronisation is of course allowed between a transmitter and a receiver, e.g. when two radio units are communicating with each other. Another rule specifies that frequency spreading must be used for a radio interface using the ISM band. The IEEE 802.11 is an example of a specification utilising the ISM band.
An example of such a radio interface is called Bluetooth (see the Telecommunications Technology Journal xe2x80x9cEricsson Reviewxe2x80x9d, No. 3 1998, with the article xe2x80x9cBLUETOOTH-The universal radio interface for ad hoc, wireless connectivityxe2x80x9d by Jaap Haartsen). Bluetooth is an universal radio interface operating within the ISM band and enables portable electronic devices to connect and communicate wirelessly via short-range, ad hoc networks (local radio networks). Bluetooth uses a frequency-hop spread spectrum technique (FH-CDMA) where the frequency band is divided into several hop channels. During a connection, radio units with Bluetooth transceivers hop from one channel to the other in a pseudo-random fashion. Each channel is divided into a number of slots in a time division multiplexing scheme, where a different hop frequency is used for each slot.
A radio unit with Bluetooth can simultaneously communicate with up to seven other radio units in a small local radio network called a piconet. Each piconet is established by a unique frequency-hopping channel, i.e. all radio units in a specific piconet share the same frequency hopping scheme. One radio unit acts as a master, controlling the traffic in the piconet, and the other radio units in the piconet act as slaves. Any radio unit can become a master, but only one master may exist in a piconet at any time (often the one that initiates the connection). It is often the radio unit that initiates the connection that acts as a master. Any radio unit may change its role from slave to master or vice versa (a slave to master or a master to slave switch) Every radio unit in the piconet uses the master identity and realtime clock to track the hopping channel. Hence the slaves must be informed of the identity and the clock of the master before they can communicate with the master.
Bluetooth supports both point-to-point (master to a slave) and point-to-multipoint (master to a number of slaves) connections. Two slaves can only communicate with each other through a master or by changing one of the slaves to a master with a slave to master switch.
There is no hop or time synchronisation between radio units in different piconets but all radio units participating in the same piconet are hop synchronised to one frequency-hopping channel and time synchronised so that they can transmit or receive at the right time. This does not contravene the rules of non synchronisation between transmitters in the ISM band because there is only one radio unit that is transmitting at any time instant in the piconet.
A radio unit can act as a slave in several piconets. This is achieved by using the time division multiplexing scheme of the channels where e.g. a first piconet is visited in a first time slot and a second piconet is visited in a third time slot. There are three different time slots on each channel where each time slot is split in two portions, one portion for transmitting and one portion for receiving.
There is no broadcast channel (e.g. a Broadcast Control Channel in GSM) in Bluetooth to which radio units that are not connected to or have not been connected to a Bluetooth piconet can listen to system information, xe2x80x9cfindxe2x80x9d a base station or to measure the signal strength/quality on.
As Bluetooth is designed to replace cables or infrared links between different electronic equipments no roaming or handover support have been incorporated in the radio interface. As soon as a radio unit connected to a piconet is moved outside the radio coverage of the master, the radio unit loses its connection (the call).
A number of problems occur when local radio networks, utilising radio interfaces that are unsynchronised with each other and have no broadcast control channel, are to be connected into and used as a cellular radio communication system.
A radio unit that is switched on in a local radio network can not be attached to the system with the help of a broadcast channel.
A radio unit that has established a link to one local radio network can not reach or be reached from another local radio network.
A radio unit can not roam or perform handover to a new local radio network when it is moved outside the local radio network it was first connected to.
The system can not measure the signal strength/quality from and keep track of neighbouring local radio networks to be able to perform high quality roaming and handover.
A radio node/base station from one local radio network can not establish a link with a radio unit in a neighbouring local radio network.
In light of the foregoing, a primary object of the present invention is to provide methods and means for creating a cellular radio communication system out of a number of local radio networks, where each network utilises a radio interface that has no broadcast channel and is unsynchronised compared to the other radio interfaces in the system. E.g. methods and means for attaching a radio unit to the system, retaining the connection to the system and providing measuring, roaming and handover capabilities.
According to a first aspect of the present invention there is a method for attaching a radio unit to a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other.
According to a second aspect of the present invention there is a method for retaining a connection to a radio unit in a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other.
According to a third aspect of the present invention there is a method for collecting data for a neighbouring list in a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other.
According to a fourth aspect of the present invention there is a method for calculating the realtime clock of a first radio node in a second radio node in a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other.
According to a fifth aspect of the present invention there is a method for co-ordinating the use of time slots in different local radio networks in a cellular radio communication system comprising a number of local radio networks utilising radio interfaces that are unsynchronised with each other.
A system according to the present invention comprises a control unit connected with a number of local radio networks and providing the basic means of a cellular radio communication system.
A control unit according to the present invention is connected with a number of local radio networks to provide the basic means for a cellular radio communication system.
An advantage with the present invention is that it is possible to attach and retain a radio unit that is switched on in the cellular radio communication system with no broadcast channel.
Another advantage is that it is possible to provide roaming and handover between local radio networks having radio interfaces that are unsynchronised with each other.
Still another advantage is that it is possible for a radio node in one local radio network to make a contact with a radio unit in another neighbouring local radio network.
Yet another advantage is that the signalling in the respective local radio network can be coordinated to facilitate inter local radio network communication.
Still another advantage is that it is possible for the system to keep track of neighbouring local radio networks to each ratio unit.